Finishing big outdoor picture frames

Based on my experience, a good quality exterior stain will usually hold up well on vertical surfaces. Where they have problems are on horizontal surfaces. So, IMO, apply an exterior opaque or semi-transparent stain. If I understand correctly, some require stripping before re-applying, but there are others that don’t.
Since you are using pine, my first concern would be that the frames are constructed so as not to trap moisture anywhere. Otherwise, they will deteriorate quickly. You might consider using pressure treated pine.

Yeah the exterior deck/fence stains with UV protection hold up well. But i’d use cedar or redwood. Not much more money but hold much better put in the elements. If they are rustic look could even use flashing on the top ledge.

Gabe, do you have access to Cypress lumber ??
a few coats of Epifanes or Pettit Marine Spar varnish will allow
the wood to carry its natural amber color.
but – is pretty time consuming for the first batch.
solution: you could make one quick set with pine in the 4 days you have
allotted for the project and work on the replacement cypress frames
with several coats of Spar at your leisure for the next project.
will last much, much longer than construction grade SYP. (and look better).

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-- I am a painter: that's what I do, I like to paint things. --